Dispensing, Compounding and Calculations
Core concepts
Accuracy in calculations and dispensing is non-negotiable. Master: dose calculations (mg/kg, BSA), dilution and concentration (C1V1 = C2V2), percentage strengths (%w/v, %w/w, %v/v), ratio strengths (1:1000), alligation, and paediatric dose checks. Understand compounding principles: accurate weighing and measuring, geometric dilution for powders, levigation, and the difference between solutions, suspensions, and emulsions. Know the requirements for extemporaneous preparations, labelling, and records. Familiarise yourself with dispensing checks: right patient, drug, dose, route, time, documentation; and the handling of controlled drugs, returns, and expiry.
Traps
Percentage strength confusion is common: 1% w/v = 1 g in 100 mL; 1:1000 = 0.1% w/v. Unit conversions (mg to g, mL to L) and drop-factor calculations cause errors. Alligation only works with like units and concentrations. Do not confuse displacement value (for suppositories) with density. Always check paediatric doses against maximums, not just weight-based formulas.
Worked example
To prepare 500 mL of 0.9% w/v saline from a 20% w/v stock: C1V1 = C2V2 → 20% × V1 = 0.9% × 500 mL → V1 = 22.5 mL stock, made up to 500 mL with diluent. Verify units and final volume.
Exam strategy
Write the equation, convert units first, then substitute. Always sanity-check the answer against the order.